C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/clientmedia.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef CLIENTMEDIA_HEADER
-#define CLIENTMEDIA_HEADER
+#pragma once
 
 #include "irrlichttypes.h"
 #include "filecache.h"
@@ -147,5 +146,3 @@ private:
 	std::string m_name_bound = "";
 
 };
-
-#endif // !CLIENTMEDIA_HEADER